The ability to precisely dispense a fluid, for example, an adhesive, is a necessity for manufacturers engaged in the packaging and plastics industries. A typical fluid dispensing operation employs a dispensing gun to apply the adhesive onto a substrate being moved past the dispensing gun, for example, by a conveyor. The speed of the conveyor, or line speed, is set according to such factors as the complexity of the dispensing pattern and the configuration of the gun. Adhesive is normally supplied to the dispensing gun under pressure by a motor driven pump.
The quality of the adhesive dispensing process is subject to many variables that include general environmental conditions, the physical state of the adhesive being dispensed, the physical condition of the dispensing apparatus and the stability of other system parameters, for example, the stability of the electrical parameters in the system. Changes in those variables often result in changes in actuation time of the dispensing gun. For example, if an electric dispensing gun is being used with an unregulated power source, fluctuations in line voltage alter the actuation time of the dispensing valve, that is, the time required to open and close the dispensing gun. An increase in line voltage results in the actuating time decreasing; the dispensing gun opening faster; and the adhesive flowing through the gun sooner than expected. Thus, the adhesive is deposited onto the substrate at a different location than anticipated. For example, upon receiving a part present signal, the gun may open so fast that the fluid is dispensed prior to the substrate reaching a position to receive the dispensed fluid. Thus, adhesive is dispensed at a location not intended to receive adhesive. A similar problem occurs if the dispensing gun experiences a drop in line voltage.
Changes in the voltage from an unregulated source may also impact the quality of the fluid dispensing process when the dispensing valve is commanded to close. Variations in gun actuation times are also caused by changes in the viscosity of the adhesive being dispensed. Heaters within the fluid dispensing system can malfunction, or heat can be transferred into, and retained by, the fluid dispensing gun in its normal operation. Either of those as well as other conditions can change the temperature of the adhesive, thereby changing its viscosity. Viscosity variations change the drag of the adhesive on the dispensing gun's armature and hence, the actuation times of the dispensing valve. As previously discussed, changes in the actuation time may result in the application of adhesive at undesirable locations on the substrate.
Variations in the operation of the dispensing gun also occur for other reasons. The mechanical wear and aging of components within the dispensing gun can impact gun actuation time. For example, a return spring is often used to move the dispensing valve in opposition to a solenoid. Over its life, the spring constant of the return spring changes, thereby changing the rate at which the dispensing valve opens and closes and hence, the location of dispensed adhesive on a substrate. Further, the accumulation of charred adhesive within the dispensing gun over its life often increases frictional forces on the dispensing valve, thereby changing gun actuation time. Thus, for the above and other reasons, the operation of the dispensing gun is subject to many changing physical forces and environmental conditions that cause variations in the actuation time of the dispensing gun. Such dispensing gun variations in opening and closing actuation times produce variations from desired locations of adhesive that are deposited onto a substrate.
There are known devices for detecting the quality of the adhesive dispensing process. Some systems attempt to monitor air bubbles and discontinuities in an adhesive bead within, or as the bead is being dispensed from, the dispensing gun. Other systems detect the presence of a bead and bead discontinuities with infrared or photoelectric sensors. Still other systems use lasers or photoelectric sensors to determine the height and/or cross-sectional area of the bead. Such systems detect physical characteristics of the dispensed adhesive bead on the substrate and hence, provide an indication of the quality of the adhesive dispensing process. While such systems effectively detect presence and size of a bead of adhesive, those systems are observing only one result of changes in the fundamental characteristics of the adhesive dispensing process.
Another system for testing for the quality of the adhesive dispensing process senses an edge of an adhesive bead within a programmed window within which the edge of the adhesive bead is predicted to occur. Such a system is a “SEAL SENTRY” monitoring system commercially available from Nordson Corporation of Duluth, Ga. By monitoring the sensed occurrences of adhesive bead edges within respective programmed windows of occurrences, the system detects bead presence and hence, provides an indicator of the quality of the adhesive dispensing process. This monitoring system requires that the adhesive pattern that is programmed into the pattern controller also be programmed into the monitoring system. Thus, the system requires a highly skilled technical person for a substantial period of time to perform the programming. Further, over a dispensing period, if the adhesive dispensing process experiences drift requiring an adjustment to the adhesive pattern in the pattern controller, it is easy to overlook the necessity of also changing the mirrored adhesive pattern in the monitoring system. Thus, this monitoring system is relatively complex, expensive and labor intensive in its programming and maintenance.
Therefore, there is a need for a monitoring system that effectively and reliably detects the quality of the dispensing process and is relatively easy for the user to setup, use and maintain.